1. Field of the Invention
The invention relates to class D amplifier circuits, and more particularly, to a method and a circuit to improve the power supply rejection ratio of a class D amplifier circuit.
2. Description of the Prior Art
Class D amplifiers are used in a variety of applications. Increasingly, MOSFET-based, class D amplifiers are used in audio applications. Referring now to FIG. 1, a simplified schematic for a class D audio amplifier is shown. A CMOS buffer B114 is used as the audio amplifier. The buffer B114 receives the input signal IN 10 and generates the output signal OUT 18. The output signal OUT 18 drives an audio speaker SPK 22, though the coupling may be indirect, such as through a capacitor.
As shown in the diagram, the CMOS buffer 14 converts the input signal IN 10 into the output OUT 18. Note that the output voltage 18 varies directly with the battery voltage BATT 26 powering the buffer 14. During the first signal pulse, the battery BATT 26 level is V1. However, during the second pulse, the battery level becomes V2. The output level OUT 18 increases due to the increase in the BATT 26 level. The energy transferred to the speaker SPK 22 by the class D amplifier is proportional to the output OUT 18 voltage multiplied by the ON time. Therefore, the energy transfer for the first pulse is proportional to the area under the signal curve (A1) while the energy transfer of the second pulse is proportional to the area A2. It can be clearly seen that the variation in battery voltage BATT 22 causes a large difference in energy transfer which translates into a large difference in speaker SPK 22 loudness. The variation in BATT 22 may be due to a large signal DC shift, as shown, or due to a large signal AC component that has been coupled onto the battery supply. Either way, the prior art circuit displays poor power supply rejection ratio (PSRR). The analysis of the simplified, prior art class D amplifier shows that either scenario may result in significant speaker SPK 22 loudness variation.
Several prior art inventions describe class D amplifiers. U.S. Pat. No. 6,107,875 to Pullen discloses a class D amplifier. A frequency compensation and gain control circuit is used to improve frequency performance. An integrator is used in the circuit. U.S. Pat. No. 6,262,632 to Corsi et al describes a class D amplifier using a ramp generator to create a delay to eliminate cross-talk. U.S. Pat. No. 5,815,581 to Andersson teaches a class D amplifier having a feedback loop. U.S. Pat. No. 5,672,998 to Wittlinger discloses a class D amplifier having a feedback loop using a pulse width modulated signal to reduce distortion.
A principal object of the present invention is to provide an effective and very manufacturable class D amplifier circuit.
A further object of the present invention is to provide a class D amplifier circuit with energy control to improve the power supply rejection ratio (PSSR).
A still further object of the present invention is to provide an energy control circuit for a class D amplifier using an analog feedback loop.
Another still further object of the present invention is to provide an improved H-bridge driver based on a class D amplifier having energy control.
Another further object of the present invention is to provide a method to improve a class D amplifier through energy control.
In accordance with the objects of this invention, an energy control circuit for a class D amplifier is achieved. The energy control circuit comprises, first, a means of generating an energy accumulation signal proportional to an output drive signal of the class D amplifier. Last, a means of receiving the energy accumulation signal and of interrupting the output drive signal when the energy accumulation signal exceeds a reference level is used.
Also in accordance with the objects of this invention, an H-bridge amplifier circuit is achieved. The circuit comprises, first, a first class D amplifier. The first class D amplifier comprises, first, a first output driver for receiving a first input signal and for generating a first output drive signal corresponding to the first input signal. Second, a first integrator generates a first energy accumulation signal proportional to the first output drive signal of the first class D amplifier. Finally, a first comparator receives the first energy accumulation signal and generates a first reset signal to interrupt the first output drive signal when the first energy accumulation signal exceeds a reference level. Second, a second class D amplifier comprises, first, a second output driver that receives a second input signal and that generates a second output drive signal corresponding to the second input signal. Second, a second integrator generates a second energy accumulation signal proportional to a second output drive signal of the second class D amplifier. Finally, a second comparator receives the second energy accumulation signal and generates a second reset signal to interrupt the second output drive signal when the second energy accumulation signal exceeds a reference level. The first output drive signal and the second output drive signal are coupled with a load therebetween.
Also in accordance with the objects of the present invention, a method to perform energy control for a class D amplifier is achieved. The method comprises, first, generating an energy accumulation signal proportional to an output drive signal of the class D amplifier. Second, the output drive signal is interrupted when the energy accumulation signal exceeds a reference level.